1. Field of the Invention
The invention relates generally to an improved compound for wallboard joints exhibiting quicker drying times and improved flexibility with enhanced strength and toughness.
2. Description of Related Art
Gypsum wallboards are formed by sandwiching a core of wet plaster between two sheets of heavy paper. When the core sets and is dried, the sandwich becomes a rigid, substantially fire-resistant unit of building material. The boards are fire-resistant because of the natural water content of the gypsum, which when exposed to heat or flame is released as steam, retarding heat transfer. Gypsum sheathing for use under exterior finishes are among the most common materials used in housing.
Interior walls are typically formed by mounting gypsum wallboards side-by-side to a frame with nails, thereby creating a joint between adjacent wallboards. To conceal the discontinuities formed by the joints, nails, and other imperfections, a joint compound is applied which subsequently dries to a hard finish. The joint compound can be sanded and smoothed such that, upon painting or papering the wall, the discontinuity is imperceptible.
Joint compounds can be broadly classified as xe2x80x9csettingxe2x80x9d type and xe2x80x9cdryingxe2x80x9d or xe2x80x9cnon-settingxe2x80x9d type. Both types are based on inorganic fillers such as clay, silica, gypsum etc. Setting types are mostly comprised of gypsum with accelerators to set the joint compound, as discussed in U.S. Pat. No. 5,653,797 issued to National Gypsum. Typical drying type joint compound compositions are based on calcium carbonate and calcium sulfate dihydrate, such as those disclosed in U.S. Pat. No. 5,336,318 issued to United States Gypsum Company.
To form a wall from the individual wallboard, first the wallboards are laid with their ends abutting. A tape is placed over the abutting ends and a joint compound is applied to the gap between the boards and allowed to dry. An embedding coating is then applied to the joint, and the embedding coating is allowed to dry. Optionally, a third coating is applied to the taped joint and more drying occurs. Finally, a finishing coat is applied to cover all the preceding coats and once again the process requires further drying. The whole process can take ten to twenty four hours of manufacturing and drying time to complete. During the periods of drying time, no work can be performed on the wallboards.
In joint compound chemistry the moisture content plays an important role in the strength of the finished material. Water is needed to hydrate the compound, but any excess water needs to be removed because excess water increases the drying time and increases the porosity of the joint compound, which reduces its strength. This is balanced against the need for sufficient water to make the compound workable before it sets. The optimum water content is an important factor affecting the strength as well as the appearance of the wallboard.
Houses using the wallboards to form interior walls either form the walls onsite or have the walls manufactured in a factory and transported to the site prefabricated. In most cases the entire house is constructed in the factory and shipped to the site. Manufacturing the wallboards at a factory significantly decreases the costs of the wallboards due to the economies of scale and the elimination of labor onsite to make the wallboards and wait for the wallboards to dry. Therefore, there is a significant advantage to manufacturing the wallboards in a factory and transporting the wallboards to the site for construction of the housing. However, during transportation of the house or the individual wallboards, vibration and stress due to packing, unpacking, and during the transport subject the wallboards to loads not experienced during normal use. There is a need for a wallboard which can be subjected to vibrational loads without cracking, and which remain flexible without losing their overall strength.
Reducing the drying time of the joint compound would permit timely application of subsequent layers of compound or paint to the wall board. Shorter drying times are of critical importance in a typical processing line used for manufacturing prefabricated walls. Also, prolonged periods of moisture on the wallboards can result in a shadowing effect, where some of the wallboard is discolored due to the water absorption. Rapid drying minimizes the shadowing effect which in turn can eliminate and/ or accelerate subsequent additional painting applications.
Another difficulty with conventional joint compounds is that they have a tendency to splatter, leading to waste, additional clean-up, and expensive precautionary measures. This problem is exacerbated where the application is on horizontal ceiling surfaces. A joint compound with improved rheology and internal cohesive strength is needed to minimize spillage and splattering of compound on the floor beneath.
The present invention is an improved wallboard compound comprising gypsum plus one or more additives to reduce the moisture in the gypsum compound and improve the strength of the finished wallboard, and a wallboard made with the improved wallboard compound. The first class of additives are water soluble polymers containing either a nitrogen (amine or substituted amines) or sulfonate group. A second class of additives are solid epoxy resins, preferably added together with the above mentioned water soluble polymer. These additives, when mixed with standard joint compound, produces a more flexible, higher strength wallboard with a reduced moisture content and quicker drying time. Further, the rheology of the compound is enhanced such that the compound has good leveling and flow behavior during application, and sufficient cohesiveness to reduce spilling or dripping from horizontally applied surface.